The biochemical oxygen demand of Douglas-fir needles and twigs, western hemlock needles and red alder leaves in stream water

Abstract

Field studies indicate that accumulation of finely divided organic debris in the channels of mountain streams after clearcutting may be responsible for subsequent reduction in the dissolved oxygen concentration. The purpose of this study was to quantify the biochemical oxygen demand (BOD) of Douglas-fir needles and twigs, western hemlock needles and red alder leaves, ascertain the chemical characteristics of the leachate from these materials, and determine if such materials are toxic to fish. Long- and short-term BOD and BOD rate constants were determined for the different types of vegetation. A standard temperature BOD test was run 90 days for the leaf material and 45 days for the Douglas-fir twigs. Four replications were run for the leaves and three run for the twigs. Half the leaf samples were fixed with a nitrification inhibitor. However, later analyses proved nitrification did not occur. As a result, all the replications within a species were combined and one curve was fitted through each group of data. A BOD curve was constructed by least squares fit through the 90 days of leaf data and through 45 days of the twig data and extended to 90 days. The 90 day K1 and the B0D90 were calculated from this curve and were: 0. 125 and 110 mg 02/gm (dry weight) for Douglas-fir needles, 0.056 and 110 mg 02/gm (dry weight) for Douglas-fir twigs, 0.064 and 166 mg 02/gm (dry weight) for western hemlock needles, and 0. 046 and 286 mg 02/gm (dry weight) for red alder leaves. Further tests showed that these 90 day values could be accurately estimated by tests of shorter duration; 20 days for Douglas-fir and western hemlock, and 60 days for red alder. The 20-day western hemlock values may, in fact, yield a more precise estimate of the actual 90-day values than the composite fit. The 20-day western hemlock projections were 0.049 for K1 and 200 mg O2Igm (dry weight) for the B0D90. Leachates of the test materials were analyzed with respect to composition, concentration, and rate of nutrient release. This information was to be used in the explanation of the 90 day BOD values obtained. Half the samples were poisoned with 2. 7 mg/l mercuric chloride to inhibit microorganism growth while the rest of the samples remained untreated to show the effect of the microorganisms. These data were of limited use due to fungal growths appearing after 20 days and failure of half the twig samples to be adequately poisoned. The test was terminated after 20 days. Simple sugars and phenolic compounds were found in all the samples. Simple sugars present were arabinose, xylose, galactose, mannose, and glucose. Mean maximum sugar concentrations released in poisoned leaf samples varied from 21.5 mg glucose equivalent/gm (fresh weight) to 15. 0 mg/gm (fresh weight) for Douglas-fir and red alder. The mean maximum sugar concentrations in the non-poisoned samples were much less: 6. 0, 5. 0, and 8. 0 mg glucose equivalent/ gm (fresh weight) respectively for Douglas-fir, western hemlock, and red alder. The rate of sugar release was rapid for Douglas-fir needles, slow for red alder, and intermediate for western hemlock. The different types of phenolic compounds were not identified. The mean maximum concentration of phenolics in the poisoned samples were 0.72, 0.46, and 0.55 mg gallic acid/gm (fresh weight) respectively for Douglas-fir, western hemlock, and red alder leaves. There was little difference between mean concentrations of the poisoned and non-poisoned samples until day 10, at which time mean phenolic concentrations of the non-poisoned samples began to drop. The Douglas-fir twigs had a slow initial sugar release, followed by an increase in the rate; the maximum concentration reached was 10. 0 mg glucose equivalent/gm (fresh weight). The pattern of phenolic release was the same with a maximum concentration of about 1. 0 mg gallic acid/gm (fresh weight). The five-day BOD of leaf material was determined under conditions of fluctuating temperature (12. 8 to 35.00 C) similar to that observed in streams exposed by clearcutting. The BOD5 values were: 202 mg 02/gm (dry weight) for Douglas-fir, 109 mg 02/gm (dry weight) for western hemlock, and 249 mg 02/gm (dry weight) for red alder leaves. The BOD5 of samples incubated at standard temperature was much lower than those exposed to conditions of fluctuating temperature; it was 25, 42, and 24 percent of the temperature fluctuated BOD5 for Douglas-fir, western hemlock, and red alder leaves respectively. The toxicity of leachate extracted from 50 grams (fresh weight) per liter of water of each species was determined on guppies and steelhead fry. The 96-hr. LC5O to guppies from leachate of Douglas-fir, western hemlock, and red alder was 35, 65, and 18 percent of the original concentration as opposed to 26, 7. 5 and 25 percent of the original concentration for the steelhead fry. These are extremely low levels of toxicity to such a concentrated sample and pose no threat to the fish.